fix discussion entry set_depth callback for rails 3

Change-Id: Ie2a1d4907f465cc665f649e85148d362e95af841
Reviewed-on: https://gerrit.instructure.com/28794
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>